no, fit to box wont resize smaller images. you could display the image in a div that is set to 560 x 420 and set the background color on the div. margin: auto; should center the smaller images.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.